Poly(ADP-ribose) polymerase 1 (PARP1) is an important cellular protein that senses DNA damage and initiates the base excision repair pathway. DNA damage (e.g., strand breaks) occurs during each cell cycle and must be repaired for a cell to survive. In cells undergoing rapid division and proliferation, such as cancer cells, levels of PARP1 are significantly increased. This is particularly true in cells lacking other DNA repair enzymes, such as cells lacking function BRCA1 and BRCA2.